1. Field of the Invention
This invention relates to the supply of cooling fluid to the rollers of hydraulically actuated roller assemblies. It relates especially, but not exclusively, to the supply to rollers that must rotate at high speed and under high loading from two opposed surfaces which contact the roller at opposite ends of a diameter, so squeezing the roller between them. The invention thus relates particularly to the rollers used in the variators of the toroidal-race rolling traction type.
2. Discussion of Prior Art
This invention has particular, though not exclusive, application to continuously-variable-ratio transmissions ("CVT's") of the toroidal-race rolling-traction type, and in particular to roller-control units of the kind by which the orientation of the traction-transmitting rollers may be controlled hydraulically, by means of the direct connection of the carriage of each roller to a piston, movable within a hydraulic cylinder connected to a controlled source of variable fluid pressure.
In such variators, the cooling of the discs and rollers and of the traction fluid is always an important consideration. As is well understood in the art, the fluid must always be present in the form of a thin film between the rollers and the toroidal races of the discs, to prevent metal-to-metal contact, so that traction is transmitted between discs and rollers by way of shear generated within the thin film. In practice the waste heat generated within the film, in an instant during which it is transmitting traction, is quickly dissipated an instant later when that particular volume of fluid will have moved clear of the "nip" between race and rollers. As to the discs, the heat conductivity of the hardened steel they are made of is not particularly high. However, the total area of each disc is high compared with the areas of instantaneous contact with its co-operating rollers, and the location of those areas of contact tends to change frequently because the ratio transmitted by the variator is also continually changing. Conventional lubrication techniques are therefore usually sufficient to prevent overheating of the discs.
With the rollers it is different, however: the heat input is always concentrated at the circumference. The concentration of the heat input is aggravated by the fact that the roller rim is, in practice, rounded to a cross-radius, so that the instantaneous "contact" between roller and race, by way of the intervening film of traction fluid, tends to be small when measured in a direction parallel to the roller axis.
British patent application number GB-A-2282196 discloses a roller assembly for a CVT in which cooling fluid is provided to the roller surface via a hollow roller support stem. The fluid is passed through a single outlet positioned opposite the roller outer surface and bathes the roller in cooling fluid.
International application no. PCT GB 97/37156 discloses, as shown in FIG. 1 of the accompanying drawings, an improved arrangement for the supply of cooling fluid to rollers in a CVT system. A roller assembly 10 comprises a roller 12 with bearings 14, 16, mounting the roller 12 for rotation on a central shaft 18 in a supporting carriage 20. The roller assembly 10 is secured to a hollow stem 22 which in turn is supported in spherical joint 24 within a piston 26 positioned within a cylinder 28. The cylinder and piston between themselves define a chamber having two portions 30a, 30b. Movement of the piston 26 is achieved by varying the pressure of hydraulic fluid supplied to each chamber 30a, 30b in a manner well known to those skilled the art and therefore not described in detail herein. Cooling fluid is supplied from a source thereof 32 to passage 34 extending along the stem 22 and thence to a pair of outlets 36 before being sprayed onto the surface of roller 12. Before passing into passageway 34 the lubricating fluid enters a chamber 38 into which the proximal end 40 of the piston/stem assembly 26,34 extends. As a consequence of this arrangement, it will be appreciated that lubricating fluid pressure will act against proximal end 40 and this might affect the accurate positioning of the roller assembly 10. A small movement effect will also be present due to the existence of surfaces 42 within passageway 34. In this arrangement, the hydraulic fluid used to control the piston position is supplied to chamber 30a, 30b from the same source 32 as that used to supply lubrication fluid to the roller.
Whilst the above-discussed arrangements provides a perfectly adequate roller control system, problems can arise in connection with the efficient supply of cooling fluid to the roller, particularly when its flow path is through the roller actuation piston. In such cases a small bore supply is often used and multiple bends are encountered before the fluid reaches the roller. The combination of these two features can cause undesirable pressure losses and reduce the effectiveness of the cooling system.